The valorization of biogas slurry through microalgal cultivation presents a sustainable approach to waste management and biomass production within the circular bioeconomy. This study investigates the potential of Nannochloropsis sp. cultivation in biogas slurry from a continuous digester, evaluating its growth kinetics, phytoremediation efficiency, and protein accumulation. The physicochemical characterization of biogas slurry revealed a dynamic nutrient profile influenced by organic load and microbial activity. Growth modeling using the Gompertz model demonstrated that optimal dilution (P2) with a C/N ratio of 6,751 supported the highest cell production rate (0,2580 day⁻¹) and a shorter lag phase (5,3175 days), attributed to balanced nutrient availability. Phytoremediation analysis indicated significant reductions in COD (75,66%), BOD (68,93%), and ammonium (83,76%), highlighting Nannochloropsis sp. as an effective biological treatment agent. Additionally, protein content in P2 (0,1412 µg/mL) closely approached that of the synthetic control medium, demonstrating its potential as an alternative nutrient source for sustainable microalgal cultivation. These findings emphasize the role of biogas slurry in microalgal-based bioremediation and biomass valorization, contributing to waste-to-product innovations aligned with circular bioeconomy and global sustainability goals.